The present invention relates to a method for producing an aluminate phosphor.
An aluminate phosphor is used in a vacuum ultra-violet radiation excited light emitting element such as a plasma display panel (hereinafter referred to as PDP) and a rare gas lamp. For instance, in a PDP, a phosphor is applied on inner walls of a multiplicity of discharge microspaces (hereinafter referred to as xe2x80x9cdisplay cellsxe2x80x9d) provided in matrix. It should be noted that discharge electrodes provided in each display cell, and a rare gas such as He-Xe, Ne-Xe, or Ar is capsulated in the space of the cell, so that discharge is caused in the rare gas when a voltage is applied to the discharge electrodes, thereby radiating vacuum ultra-violet rays. The phosphor is excited by the vacuum ultra-violet rays, thereby emitting visible rays. Here, by specifying positions of the display cells to emit light, images are displayed. Furthermore, by using phosphors emitting light of blue, green, and red colors that are three primary colors of light, full-color display can be carried out. It is difficult to increase the screen size in the case of cathode ray tubes or color liquid crystal displays, but PDPs make it possible, and further, they can be made flat panel displays. Therefore, expectations are placed on PDPs as displays in public spaces and for use in TV sets with large screens.
Apart from the PDPs, rare gas lamps for illumination are known as vacuum ultra-violet radiation excited light emitting elements. The rare gas lamp has the same configuration as the PDP having only one cell, and a phosphor is applied on inner walls of a light emitting section thereof. In the case of the rare gas lamp also, as in the case of the PDP, vacuum ultra-violet rays are generated by discharge in the rare gas, and the vacuum ultra-violet rays are converted into visible light by the phosphor, whereby light is emitted. The rare gas lamps are used for illumination, and attract attentions from the viewpoint of environment preservation since they do not use mercury, unlike the fluorescent lamps.
Known as the aluminate phosphor is, for instance, a compound containing a complex oxide expressed by a composition formula x1M1Oxe2x80x94y1MgOxe2x80x94z1Al2O3 as a substrate, and an activator. As to typical examples thereof, BaMgAl10O17:Eu, BaMgAl14O23:Eu, etc., which contain Eu as the activator and in which M1 is Ba, are known as blue light emitting phosphors, and BaAl12O19:Mn, BaMgAl14O23:Mn, etc., which contain Mn as the activator and in which M1 is Ba, are known as green light emitting phosphors.
It is also known that each of these aluminate phosphors is produced by mixing compounds in powder form that contain metal elements composing the aluminate phosphor at a predetermined ratio such that a target aluminate phosphor is generated, and calcining the obtained powder mixture in a reducing atmosphere. For instance, it is known that an aluminate phosphor expressed by a composition formula of Ba0.9Eu0.1MgAl10O17 is obtained by mixing a barium compound, a europium compound, a magnesium compound, and an aluminum compound at a ratio of Ba:Eu:Mg:Al=0.9:0.1:1:10, and calcining the obtained powder mixture in an atmosphere containing hydrogen, that is, a reducing atmosphere.
Such an aluminum phosphate is used in display cells of a PDP or the like. Since a PDP is required to have a high emission luminance, a phosphor for use in display cells of a PDP is also required to have a high emission luminance. Furthermore, a rare gas lamp for illumination is also required to have a high emission luminance, and hence, a phosphor for use in a rare gas lamp is also required to have a high emission luminance.
However, the method in which compounds containing metal elements composing an aluminate phosphor are simply mixed in powder form and a powder mixture obtained is calcined, that is, the method in which a powdery mixture is used as a precursor, has had a drawback in that a phosphor obtained does not have a sufficient emission luminance.
An object of the present invention is to provide an aluminate phosphor having a high emission luminance, a method for producing the same, and a vacuum ultra-violet radiation excited light emitting element in which the aluminate phosphor is used.
In such a situation, the inventors of the present invention, as a result of earnest studies, found that in the case where a precipitate mixture produced by using an xcex1-alumina powder, an aqueous solution containing a cation of a metal composing the aluminate phosphor, and a precipitant that precipitates the cation is used as a specific precursor to obtain an aluminum phosphor, the obtained aluminate phosphor has a significantly high emission luminance, and consequently they completed the present invention.
More specifically, the present invention is to provide an aluminate phosphor excellent in practical application that is obtained by mixing an xcex1-alumina powder, an aqueous solution containing a cation of a metal composing the aluminate phosphor, and a precipitant that precipitates the cation, and drying and calcining an obtained mixture of precipitates, to provide a method for producing the same, and to provide a light emitting element in which the same is utilized.
The present invention is to provide the foregoing method further arranged so that the aluminate phosphor contains a complex oxide expressed by a composition formula of x1M1Oxe2x80x94y1MgOxe2x80x94z1Al2O3 (M1 represents at least one metal element selected from the group consisting of Ba, Sr, and Ca, x1 satisfies 0.5xe2x89xa6x1xe2x89xa64.5, y1 satisfies 0xe2x89xa6y1xe2x89xa64, and z1 satisfies 0.5xe2x89xa6z1xe2x89xa620) as a substrate, and at least one of Eu and Mn as an activator. The present invention is to provide the aforementioned method further arranged so that the aluminate phosphor contains a complex oxide expressed by a composition formula of x2CeO1.5xe2x80x94y2M2Oxe2x80x94z2Al2O3 (M2 represents Mg and/or Mn, x2 satisfies 0.9xe2x89xa6x2xe2x89xa61.1, y2 satisfies 0.9xe2x89xa6y2xe2x89xa61.1, and z2 satisfies z2=5.5) as a substrate, and at least one of Tb and Mn as an activator.
Furthermore, the present invention is to provide an aluminate phosphor obtained by the foregoing producing method in which the aluminate phosphor comprises not less than 80 weight % a primary particle diameter of not less than 0.05 xcexcm and not more than 30 xcexcm. Furthermore, the present invention is to provide a vacuum ultra-violet radiation excited light emitting element in which the foregoing aluminate phosphor is used.
It should be noted the mean primary particle diameter is a number-average value of particle diameters determined using photographs taken by the scanning electronic microscope.
The following description will depict the present invention in more detail.
An aluminate phosphor of the present invention is characterized in that an xcex1-alumina powder, an aqueous solution containing a cation of a metal, and a precipitant that precipitates the cation are mixed, and an obtained mixture of precipitates is used as a precursor of the aluminate phosphor.
Here, examples of the cation in the aqueous solution containing the cation of a metal composing the aluminate phosphor include cations of Ba, Mg, Eu, Mn, Sr, Ca, Tb, Ce, and Zn. Examples of aqueous solutions containing such cations include an aqueous solution of a water-soluble salt formed with such a cation, for instance, a halogenide, or a nitrate. Therefore, when a blue light emitting phosphor (BaMgAl10O17:Eu, BaMgAl14O23:Eu, etc.) is produced, an aqueous solution of a water-soluble salt containing ions of Ba, Mg, and Eu may be used. Likewise, when a green light emitting phosphor (BaAl12O19:Mn, BaMgAl14O23:Mn, etc.) is produced, an aqueous solution of a water-soluble salt containing ions of Ba, Mg, and Mn may be used.
The water used for preparing the aqueous solution containing the cation in the producing method of the present invention may be any water as long as it is capable of dispersing the xcex1-alumina powder. Furthermore, the water may contain a disperser such as stearic acid, polycarboxylic acid ammonium salt, or the like.
As the xcex1-alumina powder, an xcex1-alumina powder having a mean primary particle diameter of not less than 0.05 xcexcm and not more than 30 xcexcm is preferably used.
In the case where the phosphor is applied on a back plate side of the display cells of a PDP, that is, on side and bottom surfaces of the display cells, the xcex1-alumina powder preferably has a mean primary particle diameter of from about 0.3 xcexcm to about 30 xcexcm, and more preferably, from about 0.3 xcexcm to about 5 xcexcm. If the mean primary particle diameter is excessively great, a produced aluminate phosphor also may have particles great in size, and it may be difficult to secure appropriate spaces for generating plasma in display cells that usually has a width of approximately 100 xcexcm each. On the other hand, if the mean primary particle diameter is excessively small, a produced aluminate phosphor also may have particles small in size, thereby exhibiting a dispersibility different from a conventionally used phosphor (having a mean primary particle diameter of 1 xcexcm to 5 xcexcm) when a phosphor paste is prepared. Therefore, this may make it difficult to provide the phosphor on the bottom and side surfaces of the display cells through a conventional process.
Furthermore, in the case where phosphor particles are provided on a front plate side, that is, upper surfaces of the display cells in the PDP, in addition to the side walls and bottoms of the display cells thereof, to further increase the emission luminance. In this case, the xcex1-alumina powder preferably has a primary particle diameter of from about 0.05 xcexcm to about 0.3 xcexcm, more preferably, from about 0.07 xcexcm to about 0.28 xcexcm, and further preferably, from about 0.1 xcexcm to about 0.25 xcexcm. If the primary particle diameter is excessively great, a visible light transmittance (transparency) of the phosphor on the front side may decrease, and visible light emitted from the phosphor provided on the bottom and side surfaces may be blocked. On the other hand, if the powder is excessively fine, the dispersion of the powder in the aqueous solution containing the cation may become difficult.
As the xcex1-alumina powder used in the producing method of the present invention, an xcex1-alumina powder made of alumina particles having substantially no fractured plane is preferable from the viewpoint of low aggregation.
The precipitant for precipitating the cation is preferably at least one selected from the group consisting of oxalic acid, ammonium carbonate, ammonium hydrogen-carbonate, sodium carbonate, sodium hydrogen-carbonate, ammonia, sodium hydroxide, acetic acid, and tartaric acid. Oxalic acid, ammonium carbonate, and ammonia are particularly preferably used from the viewpoint of the easily reacting with the cation and causing the precipitate mixture to be deposited on surfaces of particles of the xcex1-alumina powder. A quantity of the precipitant to be used is preferably equal to or more than a stoichiometric quantity required for causing cations other than those of aluminum cation to be deposited as precipitates, and more preferably, not less than three times the foregoing stoichiometric quantity.
The present invention is characterized in that an xcex1-alumina powder, an aqueous solution containing a cation of a metal composing the aluminate phosphor, and a precipitant that precipitates the cation are mixed, and an obtained mixture of precipitates is used as a precursor of an aluminate phosphor. Here, the mixing method is not particularly limited, but preferably used is a method of dispersing an xcex1-alumina powder in an aqueous solution containing a cation of a metal composing the aluminate phosphor, and subsequently adding a precipitant thereto while stirring. The prepared mixture of precipitates is normally dried after being separated by filtering, centrifugal separation, or another method. A drying temperature is preferably in a range of about 20xc2x0 C. to about 300xc2x0 C., and more preferably, in a range of about 90xc2x0 C. to about 200xc2x0 C. Examples of a method of directly drying the same include evaporation, and spray-drying in which the mixture is dried while it is being granulated.
Subsequently, the dried precursor is calcined, and the calcining may be carried out by any method. For instance, the precursor may be charged in an alumina boat and calcined in a predetermined gas atmosphere at a temperature in a range of about 1000xc2x0 C. to about 1700xc2x0 C., so that an aluminate phosphor is obtained.
For instance, in the case where a precursor of a blue light emitting phosphor (BaMgAl10O17:Eu, BaMgAl14O23:Eu, etc.) is calcined, it is preferable to apply a calcining operation in a reducing atmosphere at a temperature in a range of about 1000xc2x0 C. to about 1700xc2x0 C. for about 0.5 to about 40 hours at least once. The calcining temperature is more preferably in a range of about 1200xc2x0 C. to about 1550xc2x0 C. Examples of a method for obtaining a reducing atmosphere include a method in which a block of graphite is placed in a boat in which the precursor is charged, a method in which the precursor is calcined in a nitrogen-hydrogen atmosphere or in a rare gas-hydrogen atmosphere, etc. Water vapor may be contained in such an atmosphere.
In the case where a precursor of a green light emitting phosphor (BaAl12O19:Mn, BaMgAl14O23:Mn, etc.) is calcined, it is preferable to apply a calcining operation in an ambient atmosphere or an oxygen atmosphere at a temperature in a range of about 1000xc2x0 C. to about 1700xc2x0 C. for about 0.5 to about 40 hours at least once. The calcining temperature is more preferably in a range of about 1200xc2x0 C. to about 1550xc2x0 C.
After calcining, treatments such as dispersion, washing, drying, and screening may be applied to the obtained aluminate phosphor.
In the case where an xcex1-alumina powder has a mean primary particle diameter of not less than 0.05 xcexcm and not more than 30 xcexcm, an aluminate phosphor obtained by the producing method of the present invention normally contains not less than 80 weight % of particles having a primary particle diameter of not less than 0.3 xcexcm and not more than 30 xcexcm, and the distribution range of the perticles is narrower than that of the particles of aluminate phosphor prepared by a conventional method. The aluminate phosphor obtained by the producing method of the present invention has a high luminance, and since the granular variation is in a narrow range, this makes it possible to apply the phosphor efficiently in discharge microspaces such as display cells of a PDP. Therefore, in the case where the phosphor is used in display cells of a PDP, a high luminance PCP can be produced.
In the case where the xcex1-alumina powder has a mean primary particle diameter of not less than 0.05 xcexcm and less than 0.3 xcexcm, an aluminate phosphor obtained by the producing method of the present invention normally contains not less than 80 weight % of particles having a primary particle diameter of not less than 0.05 xcexcm and not more than 0.3 xcexcm, preferably not less than 80 weight % of particles having a primary particle diameter of not less than 0.07 xcexcm and not more than 0.28 xcexcm. Hence, the aluminate phosphor has a primary particle diameter smaller than that of an aluminate phosphor obtained by a conventional method, and contains fewer oversize particles. The aluminate phosphor obtained by the producing method of the present invention has a high luminance, and is suitable as a phosphor to be applied on a front panel side of display cells of a PDP since the phosphor has a small primary particle diameter. Furthermore, since it contains fewer oversize particles, it can be applied thereon efficiently.
Since an aluminate phosphor obtained by the present invention has a small primary particle diameter and contains fewer oversize particles, it particularly has excellent light emitting characteristics when excited by vacuum ultra-violet rays. Hence, it is significantly advantageous as a phosphor for use in a vacuum ultra-violet radiation excited light emitting element that can be used in various displays such as a PDP. Furthermore, an aluminate phosphor obtained by a producing method of the present invention exhibits excellent light emitting characteristics not only when excited by vacuum ultra-violet rays, but also when excited by ultra-violet rays, cathode rays, or X-rays. In other words, the present invention can be excited by a variety of energy including vacuum ultra-violet rays, ultra-violet rays, cathode rays (electron beams), and X-rays.
An aluminate phosphor obtained by the producing method of the present invention has a small cohesive power between primary particles, and hence, it allows a post-treatment such as fracture or the like to be simplified.